The present invention generally relates to thermal ink jet printers and in particular to a recording head of such a printer for projecting droplet of ink by a force of bubble created in the ink.
Non-impact recording is substantailly free from noise and is widely used in personal computers and various information processing apparatuses. Particularly, a so-called ink jet printer is used extensively because of its high speed and ease of use as this type of printer does not require specially processed paper or fixing procedure after the printing.
There are wide variety of approaches to realize the ink jet printer for actual use, some already established, some still under development.
Generally, an ink jet printer projects a droplet of recording liquid called ink so that the droplet is deposited on a recording medium such as a paper. There are several known methods to form such a droplet and to control the movement of the droplets thus formed.
In a first typical prior art method known as "TELETYPE" system disclosed in the U.S. Pat. No. 3,060,429, the droplet of ink is formed electrostatically and the movement or trajectory of the droplet thus formed is controlled by an electrical field which is changed in correspondence to a recording signal. More specifically, an electrical field is applied between a nozzle for ejecting the ink droplet and an acceleration electrode disposed in front of the nozzle. The nozzle ejects an ink droplet which is charged uniformly and the droplet thus ejected is passed through an X-deflection electrode and a Y-deflection electrode both producing a control electrical field responsive to the recording signal. Thus, the droplet is projected along a trajectory which is determined by the recording signal and arrives at a desired point on the paper.
In a second typical prior art method known as "SWEET" method disclosed in the U.S. Pat. No. 3,596,275, the droplet of ink is formed by a continuous ultrasonic vibration such that the formed droplet has a controlled electrical charge. More specifically, a piezoelectric oscillator or transducer is provided on a printer head for forming the droplet and an electrode applied with a recording signal is provided in front of an orifice of nozzle with a predetermined separation. In operation, the piezoelectric transducer is driven by an electrical signal having a predetermined frequency, and responsive thereto, the droplet of ink is formed by atomization. This droplet is ejected from the nozzle and passes through the electrode whereby the droplet is provided with an electrical charge in correspondence to the recording signal applied to the electrode. The droplets thus charged are deflected according to the amount of the electrical charge they are carrying when they pass by a deflection electrode.
In a third typical prior art method known as "HERTZ" system disclosed in the U.S. Pat. No. 3,416,153, an electrical field is established between a nozzle and a ring-shaped charging electrode, whereby atomization of ink droplet is controlled by modulating the electrical field responsive to a recording signal. According to this method, printing with gradation of recording image can be achieved.
In a fourth typical prior art method known as "STEMME" system disclosed in the U.S. Pat. No. 3,747,120, droplet of ink is ejected from a nozzle under control of a recording signal. Thus, this method is fundamentally different from those three other prior art methods in which the trajectory of the droplet is controlled electrostatically to achieve a desired printing. More specifically, the Stemme system uses a piezoelectric transducer for atomizing the ink by a mechanical vibration which in turn is caused by the recording signal.
In each of these four prior art methods, there are still various problems. For example, the first and third prior art methods need a high voltage to create the droplet, and associated therewith, there is a problem in that assembling of a number of recording nozzles in a single recording head becomes difficult. When the number of nozzle in the printer head is reduced, the speed of printing is reduced. The second prior art method, though allowing a multi-nozzle construction relatively easily, has a problem in that the construction of the recording head is complex and needs a delicate electrical control in order to achieve a desired printing result. Further, the second method has a problem in that so-called satellite dot tends to appear on the recording paper. In the third method, though capable of recording an image with excellent gradation, has a problem in that the control of atomization is difficult, the printed image tends to suffer from fog, and that the multi-nozzle construction is difficult which in turn means that the method is not suited for high speed printing.
The fourth method has various advantages over the first through third prior art methods in that the recording head has a simple construction, recovery of those droplets not used for recording can be eliminated in contrast to the first through third prior art methods, as the ink droplet is created on-demand responsive to the recording signal, and that the use of electrically conductive ink can be eliminated in contrast to the first and second prior art methods. Thereby, a wide variety of inks can be used.
This last prior art method, however, also has a problem in that the machinning of the recording head is difficult and that the miniaturization of the piezoelectric transducer having a desired resonant frequency is extremely difficult. This difficulty in turn invites difficulty in achieving multi-nozzle construction for the recording head and the printing speed of the head is inevitably reduced. Further, this method is disadvantageous for high speed printing as the droplet is created by mechanical vibration of the piezoelectric transducer.
The aforementioned U.S. Pat. No. 3,747,120 also describes a modification of the fourth prior art method in which thermal energy instead of mechanical vibrational energy is used for creating the droplet. According to the description therein, a heating coil is used for directly heating the ink to form a high pressure vapor which in turn causes pressure increase in the ink. Thus, the printer disclosed operates as a so-called bubble jet printer.
However, the aforementioned U.S. patent, while disclosing vaporization of ink in an ink chamber having a single outlet by direct heating of the ink using a heating coil supplied with current and acting as pressurizing means, is entirely silent about how to heat the ink when the ejection of ink is to be performed repeatedly. Further, the heating coil is provided at an innermost section of the ink chamber away from the outlet and thus there is a problem of complex head construction inadequate for high speed printing operation. Further, this prior art reference is silent about how to prepare for next ink jet ejection after an ink jet is ejected by the action of heat. Note that this is extremely important for actual use.
Thus, the prior art methods reviewed heretofore are unsatisfactory from the view point of high speed printing, multi-nozzle construction, appearance of satellite dots, fog in the printed image and the like, and they could only be used for limited applications where the problem inherent thereto does not cause serious difficulty.
On the other hand, the Laid-open Japanese Patent Application No. 82663/1980 describes a bubble jet printer having an improved response of ink droplet ejection and an improved temperature response of heater used therein for creating an ink vapor, wherein a part of the ink from which the vapor is to be formed is rapidly cooled by cooling a substrate holding the heater such that the temperature of the heater is rapidly cooled after ejection. According to this prior art printer, formation of bubbles due to dissolved oxygen and the like in the ink after ink droplet ejection is minimized and the speed of printing is improved. Further, the Laid-open Japanese Patent Application No. 211045/1986 discloses a bubble jet printer wherein heater and temperature detection means are provided on a printer head unit and the printer head unit is air cooled by a blower. The printer further has a controller for driving the heater and to energize the blower responsive to a signal from the temperature detection means, and as a result, the printer can maintain the temperature of the printer head unit at a temperature suitable for forming the ink droplet. However, these prior art bubble jet printers are not designed for effective heat dissipation and have to rely upon external cooling means such as large and bulky heat sink or blower provided separately from the printer head. Such a construction occupies a large space and is obviously disadvantageous for a high speed printer where a number of nozzles are provided on the printer head unit.
Meanwhile, a bubble jet printer disclosed in the Japanese Laid-open Patent Application No. 128468/1980 describes a protection layer of heater used in the printer which is chosen singularly or in combination from: a group of transitional metal oxides such as titanium oxide, vanadium oxide, niobium oxide, molybdenum oxide, tantalum oxide, tungsten oxide, chromium oxide, zirconium oxide, hafnium oxide, lanthanum oxide, yttrium oxide, manganese oxide and the like; a group of metal oxides such as aluminium oxide, calcium oxide, strontium oxide, barium oxide, silicon oxide and the like; a group of nitrides having a high resistivity such as silicon nitride, aluminium nitride, boron nitride, tantalum nitride and the like; or a group of semiconductor materials which, although having a low resistivity as a bulk, exhibits a high resistivity when formed in a thin film having a thickness of 0.1 .mu.m-5 .mu.m, preferrably 0.2 .mu.m-3 .mu.m by sputtering, chemical vapor deposition, vacuum deposition, vapor-phase reaction, liquid coating and the like such as amorphous silicon and amorphous selenium. Note that such a protective film is essential for avoiding corrosion of the heater by reaction with the ink and to avoid short circuit conduction across the ink.
Alternatively, there is proposed to cover the heater by a resin which is easily formed into film, and when formed into a film, forming a dense structure which is substantailly free from pinholes, free from swelling or dissolution even when contacted with ink, having a high resistivity when formed into film and having an excellent resistance to heat. Such material may be chosen from silicone, fluorocarbon resin, aromatic polyamides, polyimide addition polymers, polybenzimidazole, metal chelate polymers, titanate esters, epoxy resin, phtalic acid resin, thermosetting phenol resin, polyvinylphenol resin, Zirox resin, triazine resin, BT resin comprising an addition polymerized resin of triazine resin and bismaleimide, and the like. Further, the film may be formed by deposition of polyxylilene resin and its derivatives.
Alternatively, the protection film may be formed by plasma polymerization of various organic monomers such as thiourea, thioacetoamide, vinylferrocene, 1,3,5-trichlorobenzene, chlorobenzene, styrene, ferrocene, picoline, naphthalene, pentamethylbenzene, nitrotoluene, acrylonitrile, diphenylselenide, P-toluidine, P-xylene, N-dimethyl-P-toluidine, toluene, aniline, diphenylmercury, hexamethylbenzene, malononitrile, tetracianoethylene, thiophene, benzeneselenole, tetrafluoroethylene, ethylene, N-nitrosodiphenylamine, acethylene, 1,2,4-trichlorobenzene, propane and the like.
However, these materials are still unsatisfactory for use in the bubble jet printer for protecting the heater from the view point of high resistance to corrosion and good thermal conductivity. Note that good thermal conductivity is essential for the protective film of heater in order to achieve a quick response of the printer head.